There are a large number of applications for the use of paperboard. All these applications have their own specific requirements on the paperboard, thus the properties of the paperboard must differ depending on the intended end use.
Paperboard which is to be converted (e.g. coated, printed, cut, creased and folded) on high running automatic machines must have the required strength to withstand the strain and stress created during the converting. Also properties such as flatness and dimensional stability are important during converting. These properties are generally improved by increased bending resistance.
Paperboards used for graphical applications (post cards, brochures, book covers etc) should have high promotion ability. The purpose of this paperboard is generally to convey a message. Since the paperboard itself is part of the message, the appearance of the paperboard is very important. Thus, the paperboard must have good visual appearance, such as high brightness, high smoothness and high cleanliness.
Typical packaging applications for paperboards are dry food (rice, cereal etc), liquids (milk, juice, hot liquids etc), tools (spare parts etc), cigarettes, pharmaceuticals, soap etc. The packages should primarily protect the contents from the surrounding environments, i.e. there is a high protection need. The package must protect the content against impacts during handling, transportation and storing, against the pressure of stacking and extreme temperatures and moisture. Thus, paperboard used for packaging applications must fulfill general strength requirements, e.g. high bending resistance, ply bond and high tear and tensile strength. Also, the demand on print quality of premium consumer goods packages can be as high as that of luxury magazines.
The weight of a paperboard should also be as low as possible, since the cost of transportation must be taken into consideration.
Some goods such as cigarettes, chocolate, drinking water etc, are highly sensitive to taint and odour changes. Packages for such goods must thus secure the flavor of the packed product. The paperboard used must thus have high chemical purity and good values in taint and odour tests. For certain products, for example milk, light can also cause quality deterioration and the paperboard must then provide light barrier capacity.
Table 1 shows examples of important properties for a number of packaging applications.
TABLE 1Examples of important properties forsome special packaging applications.Packaging applicationProperties neededDeep-frozen foodStrength/toughness, bending resistance,compression strength, taint and odourCup StockFormability, internal sizing and structureagainst hot liquids, purity, Scott bond,printability, optical appearanceLiquid PackagingBending resistance, printability, taint andodour, purity, ply bond, optical appearance,internal sizing and structure against liquidpenetration, barrier properties
The highest quality paperboard available is made entirely of chemical pulp, e.g. SBS (solid bleached sulphate). Such paperboard has a very good appearance however needs to have a high grammage in order to give required bending resistance. This kind of solid paperboard is commonly used for packaging of e.g. cigarettes or bottle of liquors. However, packages made from this kind of paperboard are less cost-efficient due to the higher material costs. Not all applications need this extraordinary quality, and thus other paperboard types with different qualities have been developed.
A paperboard generally comprises of 1-5 plies (layers). A paperboard which consists of three or more plies comprises top and back plies, and one or more middle plies. An important property for a high quality paperboard is bending resistance, which is needed to achieve good runnability during converting (e.g. printing, creasing, cutting and forming of the package). High bending resistance promotes good runnability on the packaging machine. Bending resistance is also needed for the protective properties of a package exterior. In packages, high bending resistance promotes rigidity and strength.
Bending resistance is most easily improved by increasing grammage, since higher grammage normally means higher bending resistance. However, an increased grammage is undesirable, due to the increase in cost (cost per package). There is thus an incentive to decrease grammage while maintaining bending resistance.
Normally, chemical pulp is used in the top and back plies of the board, particularly softwood pulp which has good strength properties. The chemical pulp also gives the top and back plies good printing properties. Hardwood pulp may also be added to the outer ply to improve the surface properties. Chemical pulp normally has high purity, which is important in many applications. The middle ply of the board may contain both mechanical pulp and/or chemical pulp. Mechanical pulp, such as CTMP, is a desirable raw material, for one thing because it can be produced to a lower cost than chemical pulp. Also, mechanical pulp has a higher yield and thus a higher efficiency of raw material usage. In high quality boards, softwood CTMP is the most common mechanical pulp used for the middle ply because softwood CTMP in addition to high bulk also has long fibers that can provide good internal bonding. Chemical pulp is normally also used in the middle ply in combination with mechanical pulp, as reinforcement, due to its high strength properties. Paperboard produced based on this concept has thus high bulk with maintained strength.
Because of its ability to combine high bulk and high internal bonding, softwood CTMP is a major raw material in the production of high quality paperboard. Unfortunately, high quality softwood is available only in a limited part of the world and the softwood CTMP supply available for the production of paperboard is not sufficient in relation to the need of high quality paperboard worldwide. The use of softwood CTMP will also be less cost efficient in many countries, due to transportation costs. This is of course a major obstacle in the production of high quality paperboard products.
There is thus a need for a substitute for softwood CTMP that may be used in the production of high quality paperboard. The objective of this invention is therefore to provide a method for manufacturing of high quality paperboard, in which softwood CTMP need not be included, and which has a quality comparable to conventional high quality paperboard. This objective is achieved by the high quality paperboard as defined in claim 1.